With the optical detection of defects in a semiconductor wafer, it is also essential to take the systematic defects into account which occur due to thickness variations when photoresist is applied to the wafer. Optical devices are particularly suited to inspect the surface of wafers. As disclosed in EP 455 857 the inspection of the surface can be done, for example, by evaluating light beams reflected from the surface of the wafer.
To find macroscopic defects on semiconductor wafers, dies on the same wafer are compared in a so-called die-to-die method. Due to the use of high-precision processes, extremely uniform structures are formed on the wafer. This is why the images taken from the dies will also be identical if there are no process malfunctions which would negatively affect the formation of the dies. Any differences between two images could therefore be interpreted as a defect. A method of this type has been described, for example, in US 2004/0105578 A1. However, the comparison can only be made for such areas on the wafer which have the same sort of dies. As a result this approach is only suitable for areas with so-called productive dies. Other areas on the wafer comprising, for example, test fields or areas without structure or situated near the edge of the wafer cannot be inspected in this way. It has been found, however, that important information can be derived from these areas which can enable or facilitate an early detection of defects. Problems with the application of photoresist in particular near the edge of the wafer can be detected at an early stage because it is here that such problems appear first, and theses problems propagate from the edge towards the center as the production continues. If these areas are not inspected, the defects cannot be detected. As a result, these defects will later also occur on the productive dies which may lead to the wafer being useless.
An apparatus for inspecting a defective wafer edge with diffracted light is disclosed, for example, in JP 09269298 A. An elliptical reflector is provided having its first focus coincide with the wafer edge, and a detector in its second focus. The wafer edge is illuminated by means of a light source producing parallel coherent light directed towards the edge in the same vertical orientation as the wafer and vertical to a virtual tangent on the wafer edge. The 0-th order of the diffracted light is filtered out by an absorber plate. The diffracted light is focused by the elliptical reflector in the second focus and received there by means of a detector. However, the apparatus is realized as a separate device and cannot be integrated into an apparatus for the integral inspection of a disk-like object.
Further, JP 11351850 discloses an apparatus based on the same principle as JP 09269298 A. It is only characterized by additional sensors mounted in the vicinity of the wafer edge, and by differentiating between defects having a random orientation and defects parallel to the surface of the wafer, i.e., transverse to the rotation direction.
U.S. Pat. No. 6,587,193 further discloses inspecting the surface of a wafer wherein an illumination is selected which scans the wafer in the form of a line. The illumination line is guided across the surface of the wafer in such a way that a two-dimensional image can be created.
From U.S. Patent Application 2003/0202178 A1 a method and apparatus for inspecting a wafer are known. Herein an illumination is radiated onto the wafer in such a way that an edge of the wafer is irradiated. This serves to scan the edge of the wafer, the result of which can then be processed by an image processing unit. Defects of the wafer can be detected by comparing the acquired edge image with a stored comparison image.
U.S. Pat. No. 5,359,407 discloses an apparatus wherein the top and bottom surfaces of a flat disk are alternately partially scanned by a light beam. In contrast to the case where the top and bottom surfaces are scanned simultaneously, diffracted light from one side is prevented from getting into the detector of the other side. Each detector is always uniquely associated with one side. To scan the two sides, either two laser beams are used, or one laser beam is subdivided into two partial beams. These partial beams are moved by one or more rotary mirrors within an angular area and redirected by deflection mirrors to each of the surfaces in such a way that the area of the wafer to be inspected is crossed by the light dot of the laser beam.
U.S. Patent Application 2004/086171 discloses an apparatus and method for inspecting a substrate. The automatic apparatus comprises a first stage for carrying the substrate and a first imaging device records an image of the circumferential area of the substrate. Further, a second stage for carrying the substrate is provided, wherein a second imaging device is provided to record an image of the substrate. A plurality of individual systems is combined in one unit, which leads, however, to an effective increase in the footprint of the entire system.
U.S. Patent Application 2004/095575 discloses an apparatus for inspecting a wafer. With the aid of a first imaging device an image can be recorded of the wafer surface. A second imaging device is provided to image the top of the edge area, the wafer edge, and the bottom of the edge area. The second imaging device is mounted on a corresponding support structure of the apparatus.
U.S. Patent Application 2005/036671 discloses an apparatus for edge inspection. The wafer is placed on a stage, and a plurality of cameras is arranged in the area of the wafer edge, from which one camera images the top of the wafer edge and the other camera images the wafer edge.
No single prior art apparatus is able to perform a full inspection of a disk-like object.